The power level of an optical signal in an optical transmission system limits the distance between regenerators or amplifiers, and needs to be controlled carefully to avoid errors in the detected bits.
A signal with too high an optical power is subject to nonlinear effects in the fibre such as Self-Phase-Modulation that can seriously degrade the signal. This causes bit errors or loss of frame in the signal. These nonlinear effects are especially severe at bit rates at and above 10 Gb/s. The onset of the nonlinear degradations can be quite sharp, in that only one or two dB of increase in power level can push a signal from optimum performance to a failed state.
A signal with too low an optical power is subject to noise degradations after attenuation by the optical link.
Erbium Doped Fibre Amplifiers can cause amplitude transients when amplifying several wavelengths at once. Consider the simple example of two wavelengths. If one wavelength is removed while the pump power remains constant, then the output power at the other wavelength will increase by 3 dB. The speed of this transient is determined by the pump power and by the response of the erbium doped fibre, and is measured in microseconds.
Addition of a second wavelength causes a similar 3 dB drop in the output power of the first wavelength present.
In a wavelength division multiplex system new wavelengths commonly need to be added to systems that are in service. This may be due to an upgrade or may be caused by replacing a unit. Wavelengths also need to be removed when replacing a unit or reconfiguring the system.
Rapid changes in the power of an optical signal at one wavelength can move another signal away from its optimum power level towards too high or too low a power. Power margin must be allocated in the design of the optical system so that during a worst case transient, when combined with other worst case conditions, the data remains error free.
Allowing this margin reduces the available performance of the system, for example, reducing the maximum allowed amplifier spacings.
Various optical elements are sensitive to power changes, including receivers. Even if remaining within an appropriate static power range, rapid power transients can still cause bit errors. For example if the transient is faster than the response rate of automatic gain control in a receiver then the receiver electronics could be momentarily overloaded. These distortions can cause errors. During a transient the electrical signal, at the decision comparator will be larger or smaller than anticipated. The eye between logic levels will move, which places the decision threshold at the wrong location in the eye which causes bit errors.
Furthermore, amplitude transients can cause phase transients in clock recovery circuits that can exceed allowable jitter ranges, even to the extent of causing bit errors.
It is important that the signals carried by the wavelengths other than those being added or deleted remain error free.
It is known from U.S. Pat. No. 5,088,095 (AT & T) that gain clamping by out-of-band lasing in an optical amplifier can improve further the response to power transients in that amplifier. However, this requires a lot more pump power than a normal amplifier which is expensive to provide.
It is also acknowledged in U.S. Pat. No. 5,088,095 (AT & T) that it was known to stabilise amplifier output by detecting power changes at an amplifier input, and induce opposing compensatory changes in the pump power control circuitry.
Cooling an optical amplifier in liquid nitrogen has been shown in Journal of lightwave technology vol. 13, No 5, May 1995, pages 782-790 "Inhomogenously Broadened Fibre-Amplifier Cascades for Transparent Multiwavelength Lightwave Networks" by Goldstein et al, to allow separate saturation of the different wavelengths and so suppresses crosstalk of the power transient between wavelengths. However this is not practical for field equipment.
These three methods attempt to eliminate or minimise the transient effect of one channel on another in an optical amplified system, by improved gain control, once the transient reaches the sensitive element.